Give us a Choice with the S Meter

Flex S Meter - I'm sure there are many users that would appreciate the ability to have the S meter work as it does in a traditional analog receiver. The S meter counting empty FFT bins and so forth is fine, as is the receiver being a perfect spectrum analyzer. That aside the ability to use the S meter as a relative indication of signal strength has been a long held tradition. So how about a software switch to allow the end user to do this? 50 uv makes S9, kick in 30 db of preamp and the meter reads S9 +30, with 0 db preamp and the antenna disconnected the S meter reads S 0. Allow the end user to have an S meter the way S meters have worked for decades, not as a precise measurement tool, but a relative reading as it has been with Collins, Yaesu, Kenwood, Icom, and so forth. I am proposing a software switch to allow the user to have either the "Flex" (so called) precision S meter, or a traditional S meter as the user so chooses. Any hurrahs here! ?? Come on, I certainly can't be the only one irritated by this!

Jay, I'm curious why you chose the xverter input and not the normal antenna? Some advantage to that? What would your results have been using Ant 1 and a reasonable SSB bandwidth like 5.6 KHZ? Under your present conditions, what does the noise level sound like in headphones? Regards, Jim

The dummy load I have is a cantenna type it hears signals and noise, The BNC terminator is quieter.

My point was more about SmartSDR can read lower than S-4, if you feed it with S-0 or less.

The IARU S meter specs state values for S1, -121 dBm to S9 +10 dB, -63 dBM, my guess at an S-0 level which isn't spec'd would be -127 dBm. I'm unable to get the actual measurement lower than -135 dBm. Which since the IARU didn't spec any negative S meter signal level values below S-1, -121 dBm will just have to do.

I don't have a signal generator that can output an S-1, -121 dBm, 0.2 microvolt signal so I can't test for the lowest IARU spec'd reading.

SmartSDR probably meets the IARU Specifications.

And the noise sounds like noise in the headphones and gets quite loud if I turn the volume up all the way.

Jay, it is possible to make attenuators for very low measurements. You are probably already familiar with this but I have made 40 DB (100:1) pads with three resistors in a Pi network. The two legs are 50 Ohms and the "bridge" between the legs is 2500 ohms. You can easily make any ratio you wish this way. Jim

Against my better judgement I will enter the fray to make a few comments. These will all relate to Collins receivers. To my knowledge and recollection, there were no Collins receivers for Amateur use at least that contained a preamp. The Collins specification for s9 was 100uV as read on the signal generator which provided 50uV across the “50 ohm” input impedance. The signal generators at least in those archaic days were calibrated in open circuit voltage at the output of the generator. Great care was taken in the design to attempt to provide as linear AGC as possible in order to have accurate S meter readings as possible. Adjustments were provided to make the S meter read 0 when no signal was applied and S-9 at 100uV on the generator. Since the receiver was not without noise at no input that necessarily implies some non-linearity at that end of meter reading. So to say the S meter was inaccurate could be both correct and incorrect since it was clearly calibrated at S9 but uncalibrated at at no input just to make meter read S0. Further, since the meter depended on the AGC buss for it’s reading, the receiver bandwidth noise differences were not taken into account since these noise differences were small compared to the S9 calibration signal. Different manufactures took more or less pain to provide a measure of the received signal. Hallicrafters was known as providing generous S meter readings. Put several of the receivers mentioned in this thread and you will probable see differences between them for the same signal at the input. So what’s the point? If you like to see the meter wiggle on SSB put a VU meter on the audio and you can make it read whatever you wish and for a given RF gain setting it might be relatively accurate.
With the FRS products vs “conventional” radios we are comparing apples and oranges. Thoughts from a Collins Engineer of 32 yrs, and retired (1991) when Direct Sampled radios were just a gleam in someone’s eye.
History lesson ended.
Bob W5TX

I noticed flex has not commented much here about the request for a non SDR signal meter, but I think I can come close.The Flex architecture is much different then a non direct sampling radio.Where a traditional receiver estimates signal strength by calculating a value from the AGC voltage, the FLEX-6000 actually measures the signal power in each FFT bin.

In order For Flex to mimic a traditional receiver, it would not be a small project. Because the Flex works in such a different way, it does not share any of the same circuitry as a traditional radio, so were would we tap into for the S meter?

If we look at the layout schematics of a direct sampling receiver we can see the problem here.

Yes, all of those measurements were done using LSB on 40m at @ 7.168 MHz and all done with a manually set 500 Hz LSB filter width. While I was working on it I noticed during one of the measurement that there was a very low level signal spike visible just emerging from the noise floor. so I moved the slice just enough to remove that from the filter passband, so that all the measurements wer done on a signal free portion of the noise floor.

Great follow up, Jay.... Now I will have to see if I can duplicate your numbers.... I just can't get my head around seeing S3 or so with no input because that means it is internal noise..... Yes, it may be reading empty bins but that still adds to the noise floor and therefore defines the weakest signal that can be heard.... OK, off to the shack. Best 73, Jim

The S-Meter was never designed to be a "relative signal strength meter" from which users get the numbers for their RST reports. (BTW: The RST system was created LONG before anyone had S-meters) From a very early point, it was designed to be a calibrated signal strength meter, from which users can make precision measurements when doing antenna experiments, etc.

The S-unit is a calibrated measurement. Each S-Unit is 6 dB stronger (or weaker) than the next one. The "standard" calibration agreed by the engineers was 50mV at the antenna = S9. in modern terms, this means -73 db down from a milliwatt = S9. if you work your way down from that by 6 dB steps, you get to where your MDS is, or the noise floor of your receiver, or the level of antenna/band noise at your station. (Note the graphs posted above)

Somewhere along the way, various manufacturers began to boost the numbers of their S-Meters, or failed to calibrate them in sync with their preamps, so that a station that by measurement is only an S9 will register S9+20 dB whenever the +20 dB preamp is engaged.

This "S-Meter bloat" lead to a misunderstanding of the true meaning of S-Meter readings, and also lead to a type of "S-meter envy" where people actually compared their bloated signal strength readings on the same station as a way of bragging that "My rig is more sensitive than yours" because of a higher S-meter reading. (This was very prevalent on 11 meters in the 70's and 80's)

Now with the advent of SDR's and especially the Flex, we have a lab-quality, precision instrument that is calibrated, and synced with the preamps so that the S-meter reading is always true, and therefore helpful in giving actual comparitive readings when adjusting equipment, antennas, etc.

It wouldn't make any sense to me to dumb it down so that it is a lying S-meter like many of us had in the old CB days.

Ken, fingers got loose again. Should have been 50uV not 50mV :). As we used to say, the S-meter accuracy on some of the earlier RX were inversely proportional to the physical size of the meter itself! In SSB was it peak or average? Depends on the AGC time constant. When S-units originated, there was only AM, FM, and CW in general usage and only AM and FM really leant themselves to “meaningful” S-unit reporting. S-units are to my knowledge, not used anywhere but in amateur radio.
Bob W5TX

Yeah. I knew the mV wasn’t right when I read it in the return email. But by then several others had typed other comments. But then, I once had a receiver that it seemed to take about 50 mV to reach S9! It was deaf as a post!

In late 1977 I had a surplus R392 that I used for CW practice while preparing for my 13 WPM General test. If I remember correctly, it had a meter that read uV and VU units, selectable from the AGC or the Audio output line. (It has been a while). I loved that receiver. It was my first general coverage “digital” receiver. But I got blisters on my thumbs changing the heavy band switch 1 MHz at a time...chunk, chunk, chunk. Going from 10 Meters to 80 was literally a pain.

Yep, I'm some where between quiet rural, and rural. We have dirt roads, but no post office delivery, We're not off the grid, but mostly off road. Except for the buzzing insects it's pretty quiet, both RF and AF.

I personally would enjoy being able to select between a bar graph and the old style simulated mechanical style S-Meter on the 6400M TFT display. My ICOM IC-7700 allows me to do this. People who don't want the analog style S-meter don't need to select it. Simple

The S-3 reading on an open disconnected ANT input, is a consequence of the open jack. the SO-239 is acting as a very short, totally mismatched antenna. A quiet termination, is required. A 50 ohm composition resistor in a BNC terminator, is a lot quieter, even a 50 ohm Cantenna attached thru 3, 5 foot pieces of RG8X, and a couple of Ant switches, is a lot quieter than an open SO-239. A shorted SO-239 is also not a matched quiet termination. but it's still a very short antenna.

With all the readings and testing, I can say that I have some understanding of FLEX approach to S meter.
However, I do not know how to evaluate the signal report of other station. Yesterday, I listened FT5000MP and Flex 6600 with switching antenna by Delta switch. Same Stations with s1-s2 on Yaesu were s6–s7 with no preamp, both at 2.1khz filters.
So, I guess I will never hear a station on 6600 below S6 or so, right?
Sergey, Kn7k

No. don't be confused with the RF sampling in the FFT bins with RF from the antenna, the FFT bins are related to the pass band width readings. so that is what your seeing. As Jay pointed out a 50 ohm antenna can show lower than the bin samples on the input.A direct sampling Flex can hear well below the noise floor.

All these discussions are about summing each noise floor fft bin. Tune a single station at any band width, and a signal strength above the noise floor then the S meter will reflect that stronger signal (Math fanatics will say the noise is still there in the sum)

Stan, it does not take a fanatic to point out it is signal plus noise to noise ration.... so when giving a report it is binning noise and external noise plus signal to binning noise plus external noise ratio. Whew!!! Took a bit to get that out. HI HI 73, Jim

James, glad you got that out and I agree; that is however, a S+N/N ratio rather than signal strength (which technically is also a ratio against a fixed standard making it a fixed voltage).My point is binning noise + external noise is a finite value, aka linear value, and when added to a log value it looses significance at over 1 S unit signal over noise, much less significance on stronger signals.The result is the S meter will reflect (granted "almost") the actual signal strength and all this noise level discussion will disappear in the "grass".So, unless working with -dbm noise levels and noise mitigation activity, it is meaningless to a signal report, which in a Flex 6000, is accurately measuring a real voltage presented to the receiver.In RS(T) the S+N/N is in the 'R'eadability and the 'S'ignal is the S meter reading. As a CW contest lover, everyone is 599 regardless of meter reading and in-the-noise 5 tries for a call sign.

This statement about hearing below the noise floor is being thrown in way too often with no evidence or scientific rationale behind it. Not to mention that it doesn’t define what noise floor is being referred to- band/atmospheric noise or radio noise. The vast majority of the time the band/atmospheric noise is significantly higher and in urban settings it is always higher.

What a radio can hear, without the help of special encoding such as FT8, is determined by the higher of the band noise and the radio’s own noise. That is, if you have band noise of -100 dbm and radio noise of -150 dbm, what you can hear is limited to -100 dbm. The laws of physics apply equally to all makes and models.

The Flex has sensitivity of ~27 uV and noise floor of about -135 dbm, quite in line with other radios:

Stan, this is a great forum and I noticed, unlike other forums, there are no personal insults. OK, with that, let me say I agree 100% but let me throw a small wrench in the S meter by saying this. Let's say that you, as my neighbor, have a Bencher Skyhawk at 70 feet and I being your poor cousin, have a G5RV at 20 feet.... What signal report do we give the RG8U we just worked?? Which one is valid? So the receiving station antenna will have a profound effect upon any signal report. OK, I'm preaching to the choir here. (both 599, right?) 73, Jim

Varistor, I do not believe there is a radio on this planet that can 'hear' below the noise lever except some computer mode like FT-8. This goes for any mode that uses human interpretation like SSB, AM or CW. Just not possible for a human to do this. Very Best, Jim

It is likely I am not being so clear about what the Flex can hear,this is a part of a reply that Steve Hicks made.

When people talk about noise floor in ham radio they are generally talking about the noise level with a 500Hz bandwidth. When the panadapter is zoomed in to the max level, the bin size today is about 5.8Hz. This is a 19dB difference in noise from where a ham would say the noise floor is to what you can see on the panadapter. This means that the panadapter can see 19dB below what most hams would call the noise floor. Your ear and brain are also able to hear below the noise floor in 500Hz because of how they work. But there are limits to how well you can hear. If you've ever worked JT65 or another long-term integrating mode, you have noticed that your computer can copy signals that you cannot hear.

AndHow it possible that the panadapter is so much better than PowerSDR? The thing about PowerSDR is that it does only one FFT at the sampling rate you have selected. So let's say that you are at 192kHz sampling rate in PowerSDR. The bin size will always be 23.43Hz. When you zoom in, you are just spreading the bins available over more pixels, but you get no better resolution... Since we can see down to 1.46Hz bins in SmartSDR on a FLEX-6700, we have a 12dB visual advantage over PowerSDR. The panadapter literally sees 12dB further into the noise. By always adjusting the sampling rate in SmartSDR, we can keep showing you better and better data in the panadapter and also allow you to see wide bandwidths when you want to.

Noise Floor is as per the chart I posted above- the Atmospheric and Urban Noise at a particular Frequency... say -110dBm (about S3) @14MHz at my so called Suburban House near a mountain top near 18 TV Towers

Minimum Discernible Signal (MDS) is the weak signal that a radio can copy in a perfect environment - say -120dBm (about S1) at 14MHz with the preamps off.

So technically the Flex can hear below the Noise Floor by 10dB at my house without preamps and by 30dB with Preamps ON (about S-2.5)

However you are 100% correct, that no one will actually copy any signals below the Noise floor unless they are using a digital mode such as FT8 which can do.

So to misquote Sherwood a bit.. All modern radios hear far too well.

That said.. I have a great laugh every time someone quotes that a K3 will copy weaker signals (0.02micro volts = -140dBm) than a Flex especially since the Noise Floor on ALL HF Bands NEVER gets below -132dBm - so it copies 8dB of Noise

OK, Howard, you and a few others seem to know a great deal about the operation of the Flex so I have a basic question that has me puzzled by the display of my 6500. I have owned this for some time but still don't know how to explain it.In a nutshell, the 'S' meter in DBM does not match the level on the panoramic display with regard to noise floor. Just for example, with the antenna shorted and 6 KHZ bandwidth, SAM on 80 M, the 'S' meter reads -105.5 DBM but the average noise displayed is 130 with no preamp. With +10 DB preamp, the dosplay says 140 and with +20 DB preamp it is 150. This happens without regard to mode and selected bandwidth. It holds true for SSB at 2.7 KHZ bandwidth as it does for CW at 400 cycles bandwidth. So first, why do the values not match and second, why would the apparent noise floor seem to drop? The sound of the noise in the receiver does increase with the addition of the pre-amps. Jim

Next time you take an
inter-continental flight, ask permission to tour the flight deck. If this is
granted, take a look at the HF COMM panel in the CMU (comm management unit)
above the pilot's and co-pilot's chairs and tell me where the S-meter is.73, Adam VA7OJ/AB4OJ

S meters, we can all agree, are only on ham gear. All the time I worked at Sylvania Gov't Systems, I never once saw an S meter......nor my time in service with PRC-53s or GRC-46's...... who would have cared.??? but we in the ham community have had them in one form or another since the 30's so I guess they are here to stay. How else could we give a DX station 5 by 9 when he is just above the noise level??? 73, Jim

I guess for some people, the fictitious reading of an Analog S-Meter gives them great comfort when reporting a signal to their "Good Buddies"

But if you ever work me and ask for a signal report, I will give you a real one by just looking at my Panadapter and report that your signal is hitting -63dBm (S9+10dB) or whatever it really is in dBm..

Realistically No one really expects a S-Meter reading other than 59 or 599 anymore.. and no one has asked me for a real reading in probably 5+ years except when we are playing with new antennas.

BUT Analog Meter can be pretty Eye Candy to match the spark gap transmitter and magic tuning eye on the corner self.

Howard, only 10 over 9???? I am insulted that my SB200 to a dipole at 25 feet is not at least 40 over!!! OK, let me confess that I love the eye candy on my SX-28A best and many of my BC radios have tuning eye tubes that capture my attention while I listed to THE SHADOW !!! Best 73, Jim

Let's not forget that ham radio is supposed to be fun! Some of us are taking ourselves way too seriously. This started as a suggestion *IDEA* to include a little check box in the software to emulate a relative, old style analog S Meter should the operator want to choose to use it. Let's keep the junk science out of the idea please.

PS Stopped at Radio CIty yesterday and they fired up the IC7610, and without an antenna the S meter read S 0. I only bring this up because apparently there is a way to ignore the empty FFT bins when feeding data to the S meter.

What you are seeing on the IC7610 is a FICTIOUS Reading Calibrated to be meaningless like all ICOM analog S-Meters. It's designed to show S0 when an antenna is disconnected.... But is the signal on the antenna really -127dBm if the MDS on the radio is not -127dBm

I appreciate and welcome your thoughts, on this though you miss the point. We all know that the S Meter is a relative reading device, not a precision measurement tool. And that's OK, nothing wrong with it.

That said I do postulate that ANY measurement taken by ANY equipment and interpreted by anyone's brain is in fact fictitious! Measurements are only guesses or approximations at best per quantum physics, Check out the wave or particle experiments performed on light. For heck's sake, in the big picture scientists are debating if our universe may be emulated or even if reality can even exist without the consciousness of the soul. Yet here we are, this little spec in the universe, taking ourselves seriously? Live, love, laugh, be kind, and have fun before it ends!

So, if S Meter readings are meaningless, then what is the point of this discussion?Other than saying my S Meter/ DBM meter, is better than yours, as has been going on here. None of this will actually matter. As has been mentioned a ton of times, no one really cares about a REAL signal report beyond "Your 59 QRZ" anymore. Some people like the look of an analog meter.So, having a choice of the display in SSDR and being able to customize it, in a way that an individual might enjoy having, there's no real reason to not have choices as far as Meters. (or any other parts of SSDR as well) After all, Eye Candy does sell. jamesWD5GWY

S-Meters have a precise definition of S9=50 Microvolts @500Hz bandwidth

You are conflating Quantum Mechanics sized measurements and the Heisenberg Uncertainty Principal with real world measurements. The S-Meter and especially the Panadapter show readings on a Flex can show a fairly close to Lab Grade precision measure of the received signal.

For those who really like Eye Candy, I posted a link earlier in this thread to the Woodbox Radio Analog Style S-Meter that works perfectly with Flex and gives one the swinging needle effect so beloved by many..

@James, you must not have been following along. the reason for this discussion is that Rex has asked for a old style meter found on many older rigs were the meter is reflective of the AGC action and just bounces around meaning nothing.

In order to even make something like that work on a Flex would not be a small task because the Flex does not share any radio circuitry with non SDR radios.

@BillSorry I retract the empty FFT bins statement. But then let me say apparently there IS a way to make an SDR receiver with an S meter that reads S 0 at the practical noise floor rather than S 4 or 5. I really do not want to criticize Flex at all. I want Flex to succeed, grow, and prosper as a North American company. Heck yes. However, please don't make me have to say something when it is posited by the few as the Holy Grail. Please don't ask for ideas and then demean what customers might feel are important to them. That said, what I stand by is Flex is a very good radio, a very, very good one worthy of pride of ownership with impeccable performance. Can't it be that and have the option to click on an analog looking relative S meter like we all have used for decades? And one that doesn't read S4 with no signal tuned in? And yes a little eye candy is nice. This is my final post on this topic. ;-)

Personally, I couldn’t care less if there is a S meter or not. As it has been stated by many the S meter is useless.

That said, my view is that displaying the noise of the ADC/thermal noise that is present in any material with temperature higher than 0K is just silly. We don’t need to spend time analyzing the math behind FFT as at the end of the day the noise we are discussing is generated by the ADC and the other components in the RF path before the ADC. It’s that simple.

It’s even more silly to argue that this is some sort of an advantage and that everyone else does it wrong. Measuring and displaying the thermal noise of the ADC has no practical use unless you want to see what the exact noise floor of your own radio is.

Rex, I am sorry you feel that way because this has been a very stimulating thread in which I learned a great deal. I for one, am glad you posted it. My FT2000 will read S-0 with the input shorted and some amount, usually S-4 with the antenna connected and just listening to background noise. I think that is what you are looking for in a meter, As for representation, I think it is just a matter of graphics to go from a bar to a meter if the behavior is the same as it currently is. I have a friend with a Flex 1500 running a version of Power SDR and it has analog style meters. They honestly look great! Of course we all know the reading is meaningless but I love it! HI HI. So, bottom line, please hang in here, bear with the comments and learn along with me. Jim

I understand Rex, I'm simply asking you to think about how an S meter works on an old non SDR radio, what makes it bounce, where does it get it's signal from. Now think about how the Flex is made to work and how the Meter in the flex works. given the huge differences in the way they work I don't even know it it's possible to produce such a thing IN SSDR.

@visitar, I agree with you. The way Flex does it is the standard witch all Lab equipment uses. as Steve Hicks says. I don't know if that is right or wrong to have a ham radio work in this manor. And it confuses the hell out of customers that are not so inclined to read up on it. Flex brought SDR to ham radio in the first place, who am I to question? I'm just explaining to some folks how their Flex radios work in this regard as the question crops up time to time.

Almost all contacts I make out side of contesting people ask and give signal information. It is the standard to find out how our stations are working, witch is sort of silly considering our signals rely on propagation...anyways

This is not the standard for lab equipment. Lab equipment is measured and calibrated to compensate for the built-in error of the piece of equipment. So if Flex were lab grade, the noise profile of each and every radio would be captured and removed from the FFT calculations. In other words, just like the 7610, the S meter would read 0.

Geez already, all readings are approximations, best guesses, estimations, and all are relative! And yes quantum physics does work here, we are trying to measure electrons, those elusive little critters that have been proven to be able to be in more than one location at the same time - bi location. They can also become entangled, traverse time, transfer information faster than the speed of light, and so forth. Check out "Measurement in Science" a hard read concerning the philosophy of measurements. The fact is our collective understanding of the universe is so lacking that we don't even know what we don't know. We can't, except to measure something relative to something. I posit the relative analog S Meter reading is just as valid as any other relative measurement made digitally. But because it is relative doesn't make it not useful. I posit that all measurements are fictitious, some might be a little closer to the truth than others. If there even is real truth and real reality. Do we even know what those two things are?

I decided to make my own S-meter, calibrated, for my Icom IC-R8600.. Total cost about $5.00.. Icom gives you an output on the radio for an analog meter,, so do others as well.. The meter is S-9 when at the 12 o'clock position as shown in these 2 pictures.. With all this S-meter talk of late, it would be nice if Flex could make the software display an Analog Meter rather then a Bar type meter.. I have plenty of radios with S-Meters to play with if I get the urge to go analog, like on my R-390A... Heathkit Meets Sharper Image!! :-) And the meter is illuminated with 3 blue LED's... Looks cool at night!

Robert, must be a resolution problem,,but I'm not sure how to fix that. Can you change the resolution to something then change it back,,it may correct it? Perhaps un install the meter and re installing it? it must have not installed correctly.

SSDR software and other programs look perfect on my LG Monitor and on my Laptop,, so I dont think I need to change anything,, but might try that on my laptop... This is the first ever program that has done this in some 3 years... The S meter is neat, but not something I must have... I may uninstall the program and try a few things... But appreciate the feedback.

WOW, lots of passion on this thread :-)I never knew the idea of an S-Meter was so important to so many people.To me, it is far more important to be able to make the QSO, or hear the weakDX on SSB, than to stare at a meter all day [which by nature of how many different manufacturers are doing it differently, is a "Meaningless Index of Performance" meter.]

SO, if I go "key down" broadcasting a carrier, and 10 different people in different locations are receiving mewith different model/brand rigs, different antennas, in different locations, each one will report a different reading. Even if they were all lined up in the same field with their different rigs and antennas, they mightwell each read different. So what use is that? If they heard me at all, I am in their log book. If not, I reallydon't care what the reading was :-) And if they did log me, again, I really don't care what the reading was :-)

And furthermore, as some previous poster pointed out, if you happen to work a contest, all the logged entries look like 59 to me, I have rarely seen different, since people are moving so fast to log contacts they don't seem interesting in staring at a meter long enough to analyze, should this be a 55 or a 57 or a 54???

Are we perhaps getting all tangled up in measurebating, when it might be far more fun to make contacts?

Neal, my 45th anniversary RS Camaro has a cluster of gauges on the console for engine temperature, oil pressure, Transmission temperature and voltmeter. I could not possibly go for a spin or to the grocery store without that essential data. Same for the 'S' meter. HI HI 73, Jim

I find signal reports absolutely meaningless unless I'm engaged in a real ragchew and one of us want to compare signals with different antennas or amp on/amp off and in that case, it's extremely 'techy' to be able to quote -dBM figures. For logging I usually use the default of 59/599 as it makes no difference for award purposes, and years from now I'm not going to care that I landed the KH1 at S1 or S9, once the QSL has been received.

All good points, Rick. The meter is reasonably good for relative measurements, not so much for absolute values. Just think that even if it were 100% accurate, it would only be valid for that moment in time subject to propagation, antennas at each end, power levels, etc. Jim

Similar question. I understand the bandwidth vs S meter reading, what I don't understand is why there is a discrepancy between the S meter and the Pan-adapter values. With an extremely narrow bandwidth (~1 Hz.) the S meter reads S1 / -123 dBm and the noise level at the frequency of interest on the Pan-adapter has peaks at ~-132 and valleys at ~-136. The difference grows as the bandwidth increases. With a nominal SSB bandwidth of 2.7 KHz the S meter reads S3 / -107 dBm while the Pan-adapter remains the same as before.

In summary I noted that the noise floor on the Pan-adapter does not change as the band pass changes from very narrow to very wide and the difference between the two varies from ~13 to ~27 dBm.

So my (similar?) question - Why is there a discrepancy between Pan-adapter and S meter?.

Quick analogy: Think of the signal you want to hear as a gold watch and noise
as muddy water. I give you a bucket with several inches of muddy water and the
gold watch in the bottom. You can see a glimmer of the watch and know
it's in there somewhere (you are zoomed out on the panadapter, the noise floor
is high and you can see a signal occasionally peak out of the noise). So
you decide to see if the watch is in the bucket and you split the water between
two buckets (zoom in on the panadapter). This causes the watch to be in
one of the buckets only and now you can see it better through the mud. As
you continue pouring into more buckets, 4 then 8 then 16 buckets, the water
level goes down by half each time (3dB lowering of noise floor in panadapter).
Eventually, the watch is sticking out of the water and you can see
exactly what it is.

You'll notice that this behavior is consistent -- as you zoom out the noise
floor rises by 3dB with each zoom-out because the bucket (called the bin) has
twice the noise in it. The net-net is the amount of noise you see is
dependent on the bandwidth in which you measure it. So in order to
talk about "noise floor," hams have adopted a standard. We talk
about the noise floor in a 500Hz detection bandwidth. To measure this
"official ham noise floor level," just set the width of the filter in
your slice receiver to 500Hz and then read the meter. This is roughly
equivalent to zooming all the way in on your FLEX-6000 and the zooming out 6
times (FLEX-6500 or FLEX-6300) or 8 times (FLEX-6700) and then the number read
along the panadapter (In reality, the bin size at this point will be ~374Hz and
so the panadapter reading will be off by about 1dB).

The slice meter S-meter measures the total power of everything in the bandwidth
of the slice filter -- noise and signal -- and provides this as a single power
reading.

Another analogy might be to look at a pail of dirt spread across your living
room floor. If you look through a paper towel tube, you might tell your
wife that the dirt level is only "a tablespoon." Your wife wife
might look at the whole room and say "No!! There's an entire pail of dirt
in here!"

One final thing -- you will also hear hams say "I can see 10dB into the
noise floor" or you might look at WSJT and see that it has numbers like
-20dB meaning it sees a signal 20dB into the noise. How is this possible?
You clearly cannot see a signal below the noise floor in your panadapter,
right? Well again, the noise floor is defined as the 500Hz noise floor.
What's really going on is that they are restricting the detection
bandwidth (think narrow CW filter) and they they can hear the signal with less
noise. Your ears (brain) do this naturally on CW -- you can hear a single
tone stick out of the noise because your brain does the equivalent of an FFT,
divides it into buckets and then tells your consciousness "there's a bunch
of noise but there a tone of roughly 1000Hz in there." WSJT uses
very narros bandwidth detectors and time-averaging to see "below the noise
floor" but what's really happening is that it's reducing the detection
bandwidth, splitting the noise into buckets and then looking for signals in the
presence of less noise.

Please remember that you cannot compare noise read on the S-meter in the slice
receiver and the panadapter noise floor because of the different bandwidths of
receivers used in the slice receiver (filter) and the FFT bin bandwidth (unless they are the same based on a zoom level).